Unlimited Power with /dev/tcp

How to use /dev/tcp with bash scripting, for when you don’t have netcat/nc/socat/open-ssl available. This is about Living off the Land (LotL) and then some.
I’ll demonstrate how to make a bash smtp scanner that brute forces user names via VRFY and EXPN commands. There are some gotchyas with interactive session automation, connection handling, cleanup, utilizing word lists, and parallelization.